The present invention relates to a ventilable silencer unit and a ventilable silencer device, which have both a ventilating function to ventilate cooling air to the engine of an automobile, for example, and a silencing function to silence the noises to be radiated from the inside of the engine room or the like.
The noises of the automobile such as a car, a motor truck or an auto bus are coarsely divided into internal ones, to be felt by a driver or passenger, and external ones, to be felt by outside persons. Individual noise controls have been made from the viewpoints of improving the marketability and preventing the noise pollution.
The external noises are coarsely classified according to their sources into engine noises, drive line noises, exhaust line noises and tire noises. On the other hand, these engine noises contain not only the mechanical noises made by the engine body but also intake line noises or cooling fan noises.
Especially in recent years, the problem of external noises has risen, stressing the idle noises of a Diesel engine. Against the sound, as radiated from the engine, of a higher contribution in the external noises, there has been made a control for reducing the external noises by arranging a shielding cover 74 having sound absorbing and shielding characteristics sideways or below an engine 70, as shown in FIG. 14, to absorb or shield the noises to be radiated from the engine block or oil pan.
Where cooling air is taken into a radiator 72 from a ventilating passage 75 of a front grille 73 by a fan 71, however, the noises which are radiated forward of the vehicle from the ventilating passage 75 of the front grille 73 are loud even if the engine room 72 is surrounded by the shielding cover 74. Thus, there arises a problem that the external noises cannot be effectively reduced. Additionally, the noises raising the problem in the external noises of the engine 70 have a relatively low frequency, around 1 KHz, and a large wavelength. These noises are hard to silence by the shielding cover 74 or a sound absorbing member attached to the cover 74. As a result, they are not absorbed by the shielding cover 74, but rather are reflected. Since these reflected noises additionally leak from the ventilating passage 75, there arises a problem that the external noises are more difficult to reduce.
On the other hand, the space around the engine room of the automobile is restricted, and it is desired to make the weight as small as possible. Therefore, the ventilable silencer device for the front grille of the automobile is required to be relatively light and thin in the ventilating direction and to have a wall shape sparing the space.
In Japanese patent application Kokai publication No. 8-177456, for example, there has been proposed a silencer which has an air resistance reduced at the ventilating time by arranging the Helmholtz type silencer in the partition portion around a partitioned air passage.
However, this silencer is troubled by a problem that the Helmholtz type silencer employed is a resonator for a single frequency, and therefore it is not suited for a wide band range. Against the noises, such as the engine noises, containing frequency components of the wide band range and having a substantially homogeneous acoustic pressure distribution around the radiator or in front of the vehicle, it is necessary to provide a silencer covering a plurality of frequencies.
Therefore, the independent air passages have to be individually provided with Helmholtz type resonators of different resonance frequencies or different shapes. As a result, the entire structure is complicated, raising a problem in that the manufacturing cost is raised.
In addition, the Helmholtz type resonance frequencies are determined depending upon many factors, including the sound velocity, the diameter of an introduction hole, the number of introduction holes, the thickness of the introduction hole portion, and the volume, thereby raising the problem that the design is complicated.
Here, if the Helmholtz type resonance frequency f is expressed for the sound velocity c, the diameter d of the introduction holes, the number k of introduction holes, the volume V, and the thickness tc of the introduction hole portions:
f=(c/2xcfx80)xc3x97SQRT(C0/V),
wherein:
xcex2=xcfx80/2; and
C0=kxcfx80(2d)2/(tc+xcex2xc3x97d/2).
In the case of the Helmholtz type resonance silencer, the structure is complicated and uses a large space so that it is hard to mount in a silencer device required to be thin for the front grille or the like. Moreover, the silencing frequency is single, so it is difficult to enhance the noise reducing effect over a wide band and against the engine noises.
Where the resonance silencer is employed in the front grille or like of the automobile, moreover, it cannot be monolithically molded by a molding method, such as an injection molding method using a synthetic resin or a metal. This requires a step of fixing the silencer, manufactured at a different step and suited for the frequency of noises to be silenced, on a passage wall, thereby raising the manufacturing cost.
In Japanese Patent Application No. 10-305306, on the other hand, we have proposed a silencer device 80 in which a branch passage 82 having a fixed sectional shape is arranged in parallel with a main passage 81 and in which an entrance opening 83 and an exit opening 84 of the branch passage 82 are opened in front of and at the back of the main passage 81, as shown in FIG. 15.
This silencer device is effective for the case in which the depth in the ventilating direction can be sufficient. Against the noises containing a low-frequency band, however, there arises a problem that the silencer device is hard to apply where the depth is restricted, as in the front grille of the automobile.
On the other hand, this silencer is characterized to silence both the sound waves of a fundamental frequency determined by the length of the silencing passage and the sound waves of a frequency which is the fundamental frequency multiplied by an integer. However, this silencer is troubled by a problem that its noise reducing effect is insufficient by any means against noises such as the engine noises covering a wide frequency band from a low frequency.
The present invention has been conceived to solve the above and/or other problems, and has an object to provide a silencer unit and a silencer device which can be applied to a portion such as the front grille or bumper of an automobile, and to not take up a large area in a ventilation front, or a large depth in a ventilating direction.
Another object of the present invention is to provide a ventilable silencer unit and a ventilable silencer device, which can not only reduce noises, such as the external noises of an engine over a relatively wide frequency band, but also retain the draft, such as cooling air into the engine room, sufficiently.
A ventilable silencer unit and a ventilable silencer device for achieving the objects described above are constructed in the following manners.
1) According to the present invention, there is provided a ventilable silencer unit comprising a box body including, a ventilation portion, a silencer portion adjacent to said ventilation portion and a dividing partition located between said ventilation portion and said silencer portion,
said ventilation portion is formed of a ventilating passage opened through front end and back end of said box body.
said silencer portion being formed by closing front end and back end of said box body and including at least one partition plate to make a plurality of silencing passages of different length,
each of said silencing passages communicating with said ventilation passage through at least one opening formed in said dividing partition, and,
at least one of said silencing passages being provided with at least one passage extending partition to extend the length of said silencing passage.
These plural silencing passages are set in length to resonate with the sound waves of the target noises having a plurality of frequencies to be silenced, and are constructed to""attenuate those sound waves.
According to the present invention, the silencer is formed into the unit of the box body so that it can be arranged in the ventilating passage requiring the draft. Moreover, the silencer is provided with the plurality of silencing passages of different lengths and different resonance frequencies. Therefore, the number of frequencies of the sound to be silenced in the individual silencing passages is increased, so that the silencer can silence a remarkably wide frequency band as a whole, thereby improving the noise reducing efficiency drastically.
According to this construction, the silencer unit of the box body is provided with the plurality of silencing passages in the silencer portion of the box body and further with the partition plate and the passage extending partitions which are jointed to the box body or the dividing partition. Therefore, since the strength and rigidity of the silencer unit is raised, the silencer unit can be prevented from being deformed or broken, as might otherwise be accompanied by the vibrations or the like of the automobile.
Moreover, the partition walls of the ventilating passage are generally parallel to the ventilating direction so that they can retain a sufficient ventilating function without obstructing the draft.
2) In the ventilable silencer unit according to the present invention, said plurality of silencing passages are formed as resonance type silencing passage having two open ends.
If these resonance type silencing passages having two open ends are arranged, the silencer unit can have a plurality of resonance frequencies fan (=(nC)/(2La1)) containing the same fundamental frequency fa1 (=C/(2La1)) as that of the sound wave having a wavelength of xcexa1=2La1 twice as long as the passage length La1, so that it can silence the noises efficiently over a wide band containing the resonance frequency fan. Here, C designates the sound velocity, and n designates a natural number of 1, 2, 3, - - - , and so on.
Moreover, the resonance frequency fan to be erased depends on the passage length, but hardly on the sectional shape, so that the degree of freedom of the size of the silencing passages is enlarged to simplify the design and the manufacture.
Moreover, resonance type silencing passages having two open ends are partitioned with the passage extending partitions so that their length can be enlarged. Therefore, the silencer unit can be shortened in depth to reduce the thickness in the ventilating direction.
3) The ventilable silencer unit according to the present invention is constructed such that said plurality of silencing passages include a branch type silencing passage having only one open end.
If this branch type silencing passage having one open end is arranged, the silencer unit can have a plurality of resonance frequencies fbn (=(2nxe2x88x921)C/(4L1)) containing the same fundamental frequency fb1 (=C/(4L1)) as the wavelength four times as long as a passage length Lb1. Therefore, the noises can be efficiently silenced over a wide band containing those resonance frequencies fbn.
Especially, the branch type silencing passage can have a shorter passage length and a lower fundamental frequency than in the resonance type silencing passage having two open ends so that it can have a high effect to reduce the noises in the low-frequency range.
4) The ventilable silencer unit thus constructed can be disposed in the ventilation portion of the front grille or bumper of an automobile.
The ventilable silencer unit according to the present invention is enabled to make its depth relatively short by its characteristics so that it can be arranged and effectively used in the relatively shallow ventilation portion of the front grille or bumper of the automobile.
In this case, the arrangement of the silencer unit in the front grille and the bumper can be made by changing the silencer unit sizes and the kinds.
5) And, one of said silencing passages has the length for silencing the sound wave having fundamental frequency of the noise at the engine idle driving time of said automobile.
Especially, the external noises of the automobile raise the problem when the automobile stops in an idle run so that they can be reduced more effectively by setting the passage length of the silencing passage according to their fundamental frequency in the idle run.
6) According to the present invention, a ventilable silencer device can be formed by jointing a plurality of ventilable silencer units.
These ventilable silencer units can be expanded to a wider area by jointing them with their open faces being in the same direction.
On the other hand, the fundamental frequencies of the individual silencing passages of those ventilable silencer units can be suitably selected by selecting the passage lengths. It is, therefore, possible to provide a silencer device having a higher silencing performance.
The silencer unit according to the present invention can be especially designed to have a short depth so that it can be assembled by jointing its upper and lower faces and its side wall plates to each other, as by assembling blocks.
Where the depth is sufficient in the mounted portion, moreover, the assembly can be made by connecting the silencer units in the longitudinal direction by jointing the rear wall plate and the front wall plate. Therefore, the silencer device can be formed by combining the units into shapes matching the individual mounting place.
If the several silencer units are assembled into the silencer device, the units can be handled altogether, so that they can be easily stored, transported and mounted.
7) The ventilable silencer device according to the present invention can be disposed in the ventilation portion of the front grille or bumper of an automobile.
The ventilable silencer device according to the present invention is disposed in the narrow introduction portion such as the ventilation portion of the front grille or bumper, i.e., the portion facing the engine room of the automobile. Then, this ventilable silencer device is high in the silencing efficiency over a wide frequency band from a relatively low frequency. Therefore, the frequency component in the relatively wide range of the wide-band external noises such as the engine noises to be radiated to the outside of the vehicle is silenced to reduce the external noises remarkably.
Therefore, the silencer device can be satisfactorily mounted on the vehicle. In addition, the ventilation is easily retained so that the cooling air of the engine is efficiently introduced into the engine room.